p-Xylene Formation by Dehydrative Aromatization of a DielsâAlder

Article pubs.acs.org/JPCC

p-Xylene Formation by Dehydrative Aromatization of a Diels−Alder Product in Lewis and Brønsted Acidic Zeolites Nima Nikbin, Shuting Feng, Stavros Caratzoulas,* and Dionisios G. Vlachos Department of Chemical and Biomolecular Engineering, Catalysis Center for Energy Innovation, University of Delaware, 221 Academy Street, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: Diels−Alder cycloaddition with furans as dienes and subsequent dehydrative aromatization are potentially valuable processes for sustainable conversion of biomass-derived furans to aromatics. We have performed electronic structure calculations to investigate the catalytic activity of HY and of alkaliexchanged Y zeolites in connection with the conversion of 2,5-dimethylfuran and ethylene to p-xylene. We have used two active site settings: an active site cluster model on which we have carried out density functional theory calculations and a mechanically embedded active site cluster model on which we have performed hybrid quantum mechanics/molecular mechanics calculations with the ONIOM scheme. Even though Lewis catalyzed Diels−Alder cycloaddition has received considerable attention over the years, we show that confinement and charge transfer in zeolite catalysts play a significant role in catalysis. Both HY and alkali-Y can catalyze the aromatization of the cycloadduct through dehydration but HY is found to be far more effective. Our analysis shows that the electron withdrawing ability of the cations and the catalytic activity of alkali-Y as Lewis acids are diminished by substrate binding-induced electron density shift from the framework oxygen atoms to the cations. On account of these inductive phenomena, we show that the DMF−ethylene cycloaddition follows a bidirectional instead of normal electron flow mechanism.

1. INTRODUCTION Diels−Alder cycloaddition between a diene and a dienophile and subsequent dehydrative aromatization have recently been proposed in the highly selective conversion of 2,5-dimethylfuran (DMF) and ethylene to p-xylene (Figure 1) in the Brønsted

intriguing questions about the catalytic pathway, the ratelimiting step and the role of Brønsted acid containing zeolites.6 Using electronic structure calculations, we show that HY is only beneficial to the dehydration of the cycloadduct, an oxanorbornene derivative, and investigate the alternative of Lewis acid catalysis by considering five alkali-exchanged zeolites Y. Lewis acids are known to accelerate the Diels−Alder by closing the energy gap between the frontier molecular orbitals of the reacting partners, stabilizing the transition state. The direction of electron demand is affected by the substituents on the diene or the dienophile, while in the presence of strong Lewis acids, an electron-rich diene may turn electrophilic on account of charge transfer to the Lewis acid to which it is coordinated. Although Diels−Alder cycloaddition has received considerable attention by theoretical chemists over the years,7−14 it has not been studied in the presence of zeolite catalysts, where confinement and charge transfer from the framework to the Lewis acid center can play a significant role. Given the impact of zeolites on commercial applications, we present here detailed mechanistic studies of the conversion of DMF and ethylene to p-xylene, analyze how Lewis acid catalysis of the Diels−Alder reaction is affected by charge screening and

Figure 1. Conversion of DMF and ethylene to p-xylene.

acidic zeolites HY and HBEA.1 Interest in this synthetic route stems from the fact that p-xylene is a platform chemical in the production of polyethylene terephthalate, an important polymer in consumer products.2 The proposed process is not only more selective to p-xylene, when compared to petroleumbased catalytic reforming processes, but also sustainable, because alkylated furans, like DMF, can be produced from cellulosic biomass.3−5 Kinetic studies have revealed that under certain conditions the rate of p-xylene production is independent of the Si:Al ratio, i.e., of the density of Brønsted active sites, which raises © 2014 American Chemical Society

Received: June 17, 2014 Revised: September 26, 2014 Published: September 30, 2014 24415

dx.doi.org/10.1021/jp506027f | J. Phys. Chem. C 2014, 118, 24415−24424

The Journal of Physical Chemistry C

Article

Figure 2. (a) Location of active sitesSite I, Site II, and Site IIIin faujasite. (b) Zeolite cluster model of Site II. Atoms of the six-ring window are shown as balls. Upward arms (tubes) are part of the supercage and wall of hexagonal prism between two sodalite cages, downward legs (wireframe) represent the quadrilateral window between the supercage and the sodalite cage. (c) ONIOM cluster model with layers represented with ball-andstick (high layer), tube (intermediate layer) and wireframe (low layer).

The ONIOM cluster was divided into three ONIOM layers, which were treated at different levels of theory. In a three-layer ONIOM calculation, the total energy of the system is approximated as

inductive effects and explore correlations that could guide the design of catalysts by means of computer-aided screening.

2. METHODS The zeolite Y is a faujasite with Si:Al ratio >2.5. The most important catalytic sites in faujasite where extra-framework counterions may also reside are Site I, located in the hexagonal prism between two sodalite cages; Site II, in the hexagonal opening between sodalite cage and supercage; and Site III, close to or in a quadrilateral opening in the supercage wall (Figure 2a). As both ethylene and DMF are too large to enter the sodalite cage, it is certain that Site I does not contribute to the observed activity. Also, both the extra-framework cations and the adsorbates scarcely occupy Site III, relative to Site II, and Site III is merely specific to zeolite X.15 Thus, we assume that Site II is largely responsible for the observed catalytic activity, and it is this site that we model. Two clusters were constructed, and two sets of calculations were performed for the mechanistic studies presented in this paper. Three-layer QM/QM/MM calculations were performed with the ONIOM method using a large cluster that includes the active site and a substantial portion of the zeolite framework around it (see image in Figure 2c and below for details). We also carried out purely quantum mechanical calculations with a smaller cluster, the size of the ONIOM cluster high layer, which we used for frontier molecular orbitals (FMO) analysis and to analyze the distribution and topology of the electron density by natural bond orbitals (NBO) and Bader’s theory of Atoms in Molecules.

E ONIOM = E(real)low + E(mid)medium − E(mid)low + E(model)high − E(model)medium

The superscripts “high”, “medium”, and “low” represent high, medium, and low levels of theory, and the terms in parentheses, “model”, “mid”, and “real”, represent the active and intermediate regions and the whole cluster, respectively. The high layer of our cluster consists of the active hexagonal ring and the six surrounding 4-T rings, a total of 18 T atoms (Figure 2b, MAlSi17O24), and of the adsorbates. The medium layer models the supercage in which the reaction takes place. The rest of the cluster was treated as the low layer. The high layer was treated with the M06-2X functional, while the semiempirical PM3 method was employed for the medium layer.16 The ethylene and DMF atoms, the aluminum atom of the zeolite, and the oxygen atoms in its first coordination shell, i.e., nearest neighbors, belong in the high layer and were modeled with the 6-31G(d,p) basis set, while the rest of the high layer cluster atoms were modeled with the effective core potential basis set LANL2DZ; the cations, also part of the high layer, were modeled with the 6-31G(d,p) basis set, with the exception of Rb+ and Cs+, which were modeled with the LANL2DZ basis set. The medium layer was modeled at the semiempirical PM3 theory level and kept frozen af ter 24416



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shell (O(2) atoms) than in the actual crystal and less so with the oxygen atoms in the second coordination shell (O(4)). In both the embedded cluster ONIOM and the small cluster (i.e., isolated ONIOM high layer) calculations, the larger cations are located farther away from the plane of the six-ring, which agrees with MAS NMR observations.20 The cations that are more submerged in the framework and closer to the basic oxygen (AlO−) are bound more strongly to the zeolite, according to relative binding energies shown in Table 2. The framework−cation interaction is ionic in character as revealed by Bader topological analysis21 of the cation-framework oxygen interactions. In Table 2, we show the Laplacian of the electron density at the corresponding bond critical points. The fact that the Laplacian is positive at all framework oxygen-alkali bond critical points (BCP) signifies electron localization around the nuclei themselves and not in the region between them, which is characteristic of ionic bonds. In contrast, in the HY active site model, Bader analysis shows that the Brønsted proton is covalently bonded to the oxygen, as the Laplacian of the electron density at the corresponding BCP is negative. According to the values of the potential energy at the corresponding BCPs (cf. Table 2) and by making use of the local virial theorem, we see that the strength of the ionic bonds decreases in the order Li > Na > K>Rb > Cs. This virial theorem-based ordering is consistent with the calculated binding energies. All transition states reported in this work were confirmed by the presence of a single imaginary frequency and by intrinsic reaction coordinate calculations. All energies from the nonembedded cluster calculations are basis set superposition error (BSSE)-corrected using the counterpoise method. All calculations were performed using the Gaussian software package (Gaussian 09, revision A.2).22 NBO analysis was performed with the program NBO 3.1. Bader theory analysis was performed with the program AIM200023 and only for bonds between atoms modeled with the 6-31G(d,p) basis set.

optimization of the bare zeolite. For the modeling of the low ONIOM layer, we employed the molecular mechanics force field UFF; the atoms of this layer were kept frozen in their crystallographic positions at all times. To build the ONIOM model, a cluster of atoms containing 17 coordination spheres around Site II, a total of 314 tetrahedral atoms, was first cut out from the periodic crystal of faujasite. The dangling Si− bonds connecting the cluster to the rest of the crystal were saturated with H atoms and oriented toward the positions of the oxygen atoms in the next coordination sphere. The cluster was optimized in two stages: in the first, the terminal Si−H bond lengths were relaxed without destroying their directionality, while the rest of the cluster was being kept frozen; in the second, the rest of the cluster was relaxed with the terminal H atoms frozen in space. In the optimized geometry the terminal Si−H bonds were equal to ∼1.4 Å. Subsequently, one Si atom of the hexagonal ring was replaced by an Al atom, the charge imbalance was compensated with an exchangeable cation M (M = H, Li, Na, K, Rb, Cs), yielding a MAlSi313O520 cluster. It was then reoptimized with the positions of the 216 terminal H atoms fixed in space. This optimization procedure resulted in a geometry that is in very good agreement with crystallographic data for zeolite Y. The average T−O (T = Si,Al) bond length of 1.67 Å compares favorably with the experimental value of 1.66 Å.17−19 The distances between the counter-cations and the neighboring oxygen atoms are also in very good agreement with those determined by X-ray and neutron diffraction and by MAS NMR spectroscopy (see Table 1).19,20 The M−Al distances and the dihedral angle the O(2) atoms span with the cation are provided in Table 2. Table 1. Model and Experimental M−O (M = Li, Na, K) Separationsa experimental

ONIOM model

bond

average [Å]

average [Å]

Li(II)−O(2)

2.0324

2.09

Li(II)−O(4)

2.9718

2.95

Na(II)− O(2) Na(II)− O(4) K(II)−O(2)

25

2.29

2.9317

2.93

2.6919

2.71

K(II)−O(4)

3.0119

3.18

2.25

range [Å] 1.95− 2.28 2.79− 3.04 2.20− 2.41 2.79− 3.04 2.58− 2.85 3.03− 3.28

cluster model average [Å] 1.85 3.06 2.22 3.16 2.65 3.33

range [Å]

3. RESULTS AND DISCUSSION 3.1. Diels−Alder Cycloaddition. 3.1.1. Lewis Acid Catalysts. Even though DMF binds more strongly than ethylene, here we investigate the cycloaddition reaction in both geometries shown in Figure 3. The cycloaddition proceeds through the concerted mechanism for all systems studied in this work and, as we show in Figure 3 for the NaY example, in the transition state, the two nascent C−C σ-bonds differ by 0.01 Å. With DMF at the active site, the ONIOM and the small cluster energy barriers, shown in Table 3, do not differ by more than 3 kcal/mol. This is well within the accuracy of the computations and in general agreement with work by Auerbach and coworkers26 demonstrating that, for calculating reaction energy barriers, long-range interactions essentially cancel out, leaving an energy controlled by local electronic interactions and that relatively large, nonembedded clusters can capture the important interactions quite reliably. The computed Ea for the gas-phase, uncatalyzed cycloaddition is 24.4 kcal/mol. In heptane, the solvent used in the process of Williams et al.,6 the computed Ea is 24.5 kcal/mol, using the SMD implicit solvent model.27 Thus, from Table 3 and Table 4, we infer that the alkali-exchanged Y zeolites are not as effective Lewis acid catalysts for DMF−ethylene cycloaddtion as one might have expected; KY and RbY, however, appear to be marginally better. When cycloaddition follows ethylene-binding to the

1.81− 1.89 2.86− 3.28 2.18− 2.26 2.82− 3.57 2.56− 2.73 3.00− 3.58

a

The set O(2) comprises the nearest and the set O(4) the nextnearest neighbors of the cation.

For the small cluster calculations, we used the high layer of the ONIOM cluster, and the calculations were performed using the same functional and basis sets as for the ONIOM highlayer, described above. The small cluster was terminated with hydrogen atoms and allowed to relax at all times, except for the cupping hydrogen atoms, which were held frozen in the crystallographic positions of the framework oxygen atoms that they replace. The small active site cluster model predicts reasonably good geometries, too, with the exception of the LiY model, in which the Li cation appears somewhat more coordinated with the oxygen atoms in the first coordination 24417



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Table 2. Geometrical Parameters of Zeolite Y Site II Active Site Models, Cation Binding Energies Relative to LiY for the Embedded Cluster ONIOM Model, and NPA Charges of Cations for the Isolated High Layer of the ONIOM Modela ONIOM model model catalyst

Al-cation separation [Å]

O(2)−O(2)−O(2)− cation dihedral [deg]

HY LiY NaY KY RbY CsY

2.43 3.08 3.24 3.54 3.69 3.86

6 16 29 50 55 59

cluster model cation relative binding energy [kcal/mol]

Al−cation separation [Å]

O(2)−O(2)−O(2)− cation dihedral [deg]

cation NPA charge [a.u.]

−(1/4)∇2 ρ(rc) × 10−2 [a.u.]

V(rc) × 10−2 [a.u.]

0 25.87 46.38 55.06 60.71

2.56 3.14 3.47 3.70 3.83 3.95

5 31 44 54 59 65

0.63 0.79 0.89 0.95 -

49.29 −7.42 −4.90 −2.24 −1.60 −1.49

−59.55 −4.55 −3.08 −1.62 −1.18 −1.15

NPA analysis did not converge for Rb and Cs. Also shown are the average of the negative of the Laplacian of the electron density, −(1/4) ∇2 ρ(rc), and average of the potential energy density, V(rc), over all cation-framework oxygen bonds, evaluated at the corresponding BCPs. In the case of HY, we only consider the BCP between the Brønsted H atom and the O(2) atom to which it is bonded. a

Table 4. Calculated DMF−Ethylene Cycloaddition Energy Barriers, Ea, for Ethylene Bound to the Active Site, LUMOEth−HOMODMF Gaps, and Transition State Charge Change, Δq‡, of DMF and Ethylene Moieties, As Defined in the Texta

a

Figure 3. Diels−Alder cycloaddition transition state geometries for ethylene (left) and DMF (right) bound to the active site (bond lengths in Å).

model catalyst

Ea [kcal/mol]

LUMOEth−HOMODMF [a.u.]

Δq‡(DMF) [a.u.]

Δq‡(eth) [a.u.]


26.1 23.3 21.6 24.3 25.4 26.2

0.280 0.279 0.267 0.283 0.285 0.286

0.205 0.188 0.164 -

−0.216 −0.192 −0.170 -

All energies are zero-point corrected.

of +0.79 and +0.89, respectively, whereas K+ is less so, carrying charge of +0.95 (see Table 2). This is consistent with the fact that the smaller cations are more submerged in the framework and thus more strongly interacting with the framework. Upon binding, there is electron density transfer from the adsorbate to the alkali cation and the adsorbate acquires a small, overall positive charge (cf. q(DMF) in Table 5 and q(eth) in Table 6). This charge transfer correlates with the binding strength. Moreover, substrate binding induces f urther electron density transfer from the framework oxygens to the cation. The total charge on ethylene bound to LiY, q(eth), is +0.07, which corresponds to the charge shift from the adsorbate to the cation. However, the Li charge differential upon substrate binding, Δq(Li), i.e., the cation’s charge change before and after binding is −0.15, which is more than the charge transfer from bound ethylene. For NaY, q(eth) = +0.03 and Δq(Na) = −0.08; for KY, q(eth) = +0.015 and Δq(K) = −0.05 (cf. Table 6). In the case of DMF binding, Δq = −0.18, −0.15, and −0.11

active site (Table 4), the lowest Ea are 21.6 kcal/mol for NaY and 23.3 kcal/mol for LiY. So, for both binding geometries, we have rather minor catalytic activity. What is quite intriguing, nevertheless, is that the “best” catalysts vary with the order with which the reactants bind to the active site. The smaller cations appear more active when the dienophile is bound to the active site, whereas the larger and less submerged in the framework cations appear more effective when the diene is at the active site. In order to investigate the reasons behind the rather unremarkable activity of alkali exchange cations in Y, it is instructive to consider the distribution of the electron density before and after binding of the reactants. Natural population analysis shows that already before substrate binding, the smaller cations Li+ and Na+ are significantly screened, carrying charges

Table 3. Calculated DMF−Ethylene Cycloaddition Energy Barriers, Ea, with DMF Bound to the Active Site, LUMOEth− HOMODMF Gaps, and Transition State Charge Change, Δq‡, of DMF and Ethylene Moieties, As Defined in the Texta

a

model catalyst

Ea,ONIOM [kcal/mol]

Ea,cluster [kcal/mol]

LUMOEth−HOMODMF [a.u.]

Δq‡(DMF) [a.u.]

Δq‡(eth) [a.u.]


25.4 25.6 24.8 21.9 21.5 23.1

26.3 26.5 21.7 22.1 21.9 22.6

0.327 0.335 0.339 0.337 0.337 0.334

0.028 0.040 0.038 -

−0.017 −0.017 −0.028 -

All energies are zero-point corrected. 24418



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Δq‡(eth) = −0.19 when ethylene is bound to the cation, whereas Δq‡(DMF) = 0.04 and Δq‡(eth) = −0.02 when DMF is bound to the active site. It thus appears that the inductive effects of the framework diminish the Lewis center’s ability to withdraw electron densityin particular from DMF, which remains nucleophilic and thus cycloaddition is nearly electron neutral, following a bidirectional electron flow mechanism.28 In gas-phase cycloaddition, FMO theory correlates low activation energies with small highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO− LUMO) gaps between the addends.29,30 Does this correlation persist when the reaction takes place at the active site of a zeolite? In Table S1, we provide the energies of the proper symmetry occupied and unoccupied molecular orbitals that are relevant to the catalyzed Diels−Alder reaction, per Woodward and Hoffmann’s theory on conservation of orbital symmetry.31 They are not strictly the HOMO and the LUMO, as there is orbital mixing and reordering due to substrate binding; this is perfectly acceptable and within the tenets of FMO theory.32 In the following, we shall be making use of the terms HOMO and LUMO, but it should be understood that we are referring to the proper symmetry orbitals. On the zeolitic model catalysts, when ethylene binding to the active site is followed by cycloaddition, then according to the HOMOeth−LUMODMF gaps obtained from the small cluster calculations (Table S3), the catalytic activity should follow the order NaY > LiY > KY ≈ RbY ≈ CsY. We see that ethylene interacting directly with the Lewis acid site becomes more electrophilic than in the gas-phase and reactivity correlates with the FMO gap. When, instead, DMF is bound to the active site, the calculated catalytic activities are incongruous with the corresponding FMO gapsthis is critical, considering that DMF and not ethylene is more likely to be bound to the active site in this catalytic process. The inequality |HOMODMF− LUMOeth| < |HOMOeth−LUMODMF| (cf. Table S3) between frontier orbitals s suggests that the reaction should follow normal electron flow. The HOMODMF−LUMOeth gaps span a range of ca. 3 kcal/mol (Table S3), which is indicative of how close the alkali-Y zeolites are in catalytic activity, also reflected in the calculated activation energies. Nevertheless, it is worth pointing out that, even within the small range of HOMODMFLUMOeth gaps, the predicted catalytic activities should drop in the order LiY ≈ CsY > KY ≈ RbY > NaY when, in fact, the computed activation energies increase in the order NaY < RbY < KY < CsY < LiY. Clearly, a simple correlation does not exist and the disagreement between the FMO predictions and the computed activation energies is rather striking. This lack of correlation does not imply that FMO fails in this instance, but rather that additional second-order−and possibly higher-order terms−in Fukui’s perturbative expansion of the energy and wave function must be considered.33−35 Indeed, we have found that the calculated activation energies are better captured by the expression

Table 5. DMF Binding Energies, NPA Charges (q) of the Cations Prior to DMF Binding and of Gas-Phase DMF, and Charge Change on the Cations (Δq) Attendant upon DMF Bindinga model catalyst

binding energy [kcal/mol]

q (cation) [a.u.]

Δq‡ (cation) [a.u.]

q(DMF) [a.u.]


8.1 18.3 14.9 12.8 9.6 7.7

0.63 0.61 0.745 0.837 -

0.001 −0.179 −0.149 −0.112 -

0.011 0.076 0.044 0.039 -

a

Geometries for binding of DMF on LiY and CsY are shown in Figure S1, a and b, respectively. All energies are zero-point corrected.

Table 6. Ethylene Binding Energies, NPA Charges (q) of the Cations Prior to Ethylene Binding and of Gas Phase Ethylene, and Charge Change on the Cations (Δq) Attendant upon Ethylene Bindinga model catalyst

binding energy [kcal/mol]

q(cation) [a.u.]

Δq(cation) [a.u.]

q(eth) [a.u.]


6.8 11.2 10.6 8.2 7.0 5.0

0.613 0.638 0.817 0.895 -

−0.016 −0.151 −0.078 −0.054

0.007 0.070 0.030 0.015 -

a

Geometries obtained for ethylene bound on LiY and CsY are shown in Figure S1, c and d, respectively. All energies are zero-point corrected.

for Li, Na, and K, respectively, implying stronger inductive effects. In this respect, an important note to add is that the charge of bound DMF, in each of these cases, is very similar to that of bound ethylene. So, substrate binding induces electron density shift from the framework oxygen atoms to the cation and this shift has a profound effect on the Lewis catalytic activity of these materials (vide inf ra). Adsorption at the HY active site is not accompanied by such inductive phenomena: the hydrogen atom’s positive charge is largely unaffected by adsorption of either addendthe H charge differentials upon ethylene or DMF adsorption are −0.016 and 0.001, respectively. Although we shall return to the HY case in the next subsection, we should note that the transition states identified in HY do not involve proton transfer to any of the reactants. In fact, no cycloaddition transition states have been located between protonated DMF and ethylene or protonated ethylene and DMF; HY is catalytically inactive for cycloaddition chemistry. It is also of interest to consider the Diels−Alder transition state charge change, Δq‡, on the DMF and ethylene moieties, which is a measure of charge transfer between the reactants (cf. Table 3 and Table 4). We find that, in all cases, Δq‡(DMF) > 0, Δq‡(eth) < 0 and that the charge of the cations is largely unaffected (cf. Table S2). So, according to Δq‡, ethylene bound to the Lewis site acts electrophilically, and cycloaddition appears to follow normal electron demandas expectedbut DMF bound to the Lewis site does not become electrophilic. We note, however, that during cycloaddition the charge transfer from active-site-bound DMF to the ethylene molecule is significantly less than from DMF to active-site-bound ethylene: in the case of NaY, for example, Δq‡(DMF) = 0.188 and

Ea = a0 + a1/x + a 2 /y

(1)

where x = HOMOD−LUMOA and y = LUMOD−HOMOA; the subscript D (A) stands for electron donor (acceptor). In FMO language, the terms x and y correspond to single-excitation configuration interaction contributions to the total wave function. The activation energy values predicted by eq 1 are depicted in Figure 4. The x-term captures the orbital overlap that leads to the formation of two new, distinct C−C σ-bonds. However, during reaction, the π bond of the dienophile breaks, 24419



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is thermoneutral. In essence, these are side-reaction channels and are outside the scope of this study. Proton transfer to the ethylene is much slower, requiring, according to our calculations, 23.8 kcal/mol of activation energy (details in Figure S2), after which polymerization ensues. The polymerization of alkenes in HY has been the subject of experimental and computational studies, which have concluded that the protonation is the rate-limiting step. Depending on reaction conditions, the activation energy ranges from 24.4 to 27.4 kcal/mol.36 Computational studies, at the ONIOM3(MP2/6-311++G(d,p):HF/6-31G(d):UFF theory level, have estimated the proton transfer barrier at 30.1 kcal/ mol.37 In the present study, we have not been able to locate a transition state for cycloaddition between DMF and protonated ethylene and, thereby, we conclude that Brønsted acid catalysis of the Diels−Alder reaction is not possible. 3.2. Dehydrative Aromatization. Turning our attention to the dehydration of the oxanorbornene derivative to p-xylene, we first note that the reaction cannot proceed uncatalyzed, as the requisite oxygen bridge opening requires more than 60 kcal/mol of activation energy.38 The reaction, however, is orders of magnitude faster over the Brønsted acid containing HY. Table 7 shows the activation energies from calculations using the ONIOM and small cluster models; the agreement between the two models is very satisfactory as the activation energies do not differ by more than 2 kcal/mol. Figure 5a shows the mechanism for the Brønsted-acid catalyzed dehydration. A fast proton transfer from the active site to the bridge oxygen atom initiates the reaction (Step 1). The transferred proton forms a hydrogen bond with the donor oxygen atom (H-bond length 1.62 Å). The oxanorbornene oxygen bridge, C1−O, breaks with activation energy of only 12.2 kcal/mol−a dramatic decrease compared to the uncatalyzed reaction. This is followed by water formation via proton transfer from the C6 carbon to the oxygen atom, requiring only 7.8 kcal/mol of activation. Finally, a second proton transfer from the C5 carbon to an active site oxygen adjacent to the Al atom completes the formation of p-xylene and regenerates the Brønsted site. We note that the Haccepting framework O atom, in Step 4, is not the one that donated the hydrogen atom in Step 1. According to our calculations, Lewis acid catalysts are also capable of accelerating the dehydrative aromatization, but they are not a match for the Brønsted acid catalyst. The calculated

Figure 4. Fitted activation energies, from equation Ea,fit = a0 + a1/x + a2/y, plotted versus the DFT-calculated activation energies, Ea, for DMF−ethylene cycloaddition. Fitted parameters: a0 = 56.3, a1 = 13.6, and a2 = 6.81, when ethylene binds to the active site, and a0 = 752.2, a1 = 187.6.1, and a2= −61.71, when DMF does; data pertaining to this plot can be found in Table S3.

and a new π bond forms in the diene moiety. In the transition state, the π bond will be delocalized between the two moieties, and the orbital that describes this delocalization is mostly the superposition of the π in the dienophile (its HOMO) and the π1* + π2* in the diene (its LUMO); the y-term in eq 1 captures this very contribution. A rigorous, theoretical justification for the inclusion of this term can be found in Fukui’s mathematical formulation of the FMO theory;34,35 a more intuitive but equally illuminating discussion can be found in Woodward and Hoffmann’s seminal paper on the conservation of orbital symmetry.31 In the zeolite, the y-term is comparable in magnitude to the x-term and thus cannot be neglected. So, in Lewis acidic zeolites, the DMF−ethylene cycloaddition is both HOMOdiene- and LUMOdiene-controlled. This is clearly the result of inductive effects attendant upon binding, which are stronger with DMF at the active site, leading to the bidirectional electron flow alluded to by the transition state charge changes Δq‡. 3.1.2. Brønsted Acid Sites. Here we consider proton transfer to either of the two addends. Protonation of DMF at either the α- or β-position of the ring is facile and leads to the formation of resonance structureswith no aromatic characterthat cannot participate in [4 + 2] cycloaddition. The activation energy for proton transfer from the Brønsted site is respectively 6.0 and 5.8 kcal/mol. Protonation at either of the two positions

Table 7. Reaction Barriers, Ea, and Energies of Reaction, ΔEr, along the Cycloadduct Dehydration Pathway on Different Catalysts (See Figure 5 for Mechanisms)a Step 1: cycloadduct rotation [kcal/mol]

Step 2 [kcal/mol]

Step 3 [kcal/mol]

Step 4 [kcal/mol]

zeolite active site model

Ea

ΔEr

Ea

ΔEr

Ea

ΔEr

Ea

ΔEr

none HYONIOM HYCluster LiYCluster NaYCluster KYCluster RbYCluster CsYCluster

3.0 2.1 2.7 1.4 4.0 2.5

−7.1 −18.1 −18.8 −11.0 −11.4 −6.8

62.8 12.2 11.3 38.2 45.0 50.6 52.0 53.5

3.4 5.6 5.7 −1.3 3.1 1.7 4.6 4.2

48.9 7.8 6.7 38.3 40.9 42.8 43.1 43.0

−5.2 −18.6 −21.0 −2.0 −7.9 −10.9 −12.5 −10.3

47.8 8.9 9.4 42.8 45.3 48.0 49.3 52.8

−20.9 −26.2 −25.0 −23.9 −27.0 −24.5 −21.1 −23.2

a

Indices ONIOM and Cluster refer to results obtained from the ONIOM QM/QM/MM model and the cluster calculations, respectively. Note that there is no cycloadduct re-orientation in the HY case. All energies are zero-point corrected. 24420



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Figure 5. Mechanism for the dehydration of the cycloadduct, 1,4-dimethyl-7-oxabicyclo[2.2.1]hept-2-ene, in HY (a) and in MY, M = Li, Na, K, Rb, Cs (b). The associated energetics can be found in Table 7. Ground and transition state geometries for the NaY-catalyzed dehydration are shown in Figure S3.

Figure 6. Reaction barriers plotted versus the values of the electron density, ρ(rc) (filled circles, lower x-axis scale), and of the negative of the Laplacian of the electron density, −∇2ρ(rc) (open triangles, upper x-axis scale), at the bond critical points, rc, of the bonds being cleaved during cylcoadduct dehydration (mechanism in Figure 5). (a) Breaking of the C1−O bridge, common to both the Brønsted acid and the Lewis acid mechanisms in Figure 5. (b) Breaking of the C3−O bond (for Lewis acid catalysis mechanism in Figure 5b). (c) Proton transfer from C5 to C4−OH and water formation (for Lewis acid catalysis mechanism in Figure 5b). Insets in panels a−c depict the corresponding elementary steps and the breaking bonds are marked with open circles.

reaction barriers using the small cluster model are provided in Table 7. Mechanistically, catalysis can take place only when the cycloadduct coordinates with the cation through its heteroatom. When ethylene adsorption precedes cycloaddition, the

cycloadduct needs to rotate in order for the O atom to coordinate with the cation (Figure 5b, Step 1); when DMF adsorbs first, then the cycloadduct does not need to reorient itself. Once the oxanorbornene derivative is properly 24421



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Figure 7. Energy profiles for the conversion of DMF and ethylene to p-xylene: uncatalyzed, black line; HY, dashed line; NaY, gray line. Reference energies for DMF and ethylene (Step 0) and p-xylene and water (Step 14) were calculated at infinite separation in the gas phase. * Steps 4 and 5 are unique to HY.

quite reliable activation energies when compared with embedded cluster, hybrid methods (e.g., ONIOM), despite the fact that long-range, electrostatic interactions with the framework of the zeolite are not considered. This is in agreement with previously published work.26 One caveat that should be added, however, is that substrate binding enthalpies will not be captured quantitatively and only trends will be reproduced, as long-range, electrostatic interactions play an important role in adsorption in zeolites. Turning our attention to the physical systems at hand and consolidating our analysis, we argue that the conversion of DMF and ethylene to p-xylene starts by preferential adsorption of DMF at the active site of the zeolite. This is consistent with the experimental observations when the reaction is carried out over HY at 900 psig and 300 °C − if ethylene were to preferentially bind to the active site, then Brønsted acidcatalyzed ethylene oligomerization would ensue.39,40 Such polymerization has not been observed, despite the fact that proton uptake by ethylene bound to the Brønsted site is faster than the cycloaddition. Proton uptake by DMF bound to the active site is fast, leading to side reactions, but it is also thermoneutral. In conjunction with the fact that the calculated cycloaddition barrier is higher in HY than in the gas phase (cf. Table 3), we maintain that, in HY, the Diels−Alder reaction proceeds essentially uncatalyzed. In fact, we believe that, under the experimental conditions used by Williams et al.,6 the cycloaddition takes place primarily in the solution phase (heptane). Dehydration of the cycloadduct is very unlikely in the absence of a catalyst, as the calculated rate-limiting step requires ca. 62 kcal/mol of activation; it is, however, very fast in HY, where breaking the C−O−C bridge (the slowest step) requires only 12.2 kcal/mol of activation energy. Thus, one

coordinated to the active site, the reaction proceeds with the breaking of the C1−O bond and formation of the C3−O bond and the C1−C3 double bond (Step 2). Two subsequent intramolecular proton transfers complete the conversion to pxylene, with concomitant water release (Steps 3 and 4). In contrast to the cycloaddition, the dehydration entails breaking and forming of a series of bonds. Brønsted and Lewis acid catalysts function alike in this situation: they weaken the bond that is to be cleaved by depleting it of electron density. This is confirmed by Bader theory21 topological analysis of the electron density. In Figure 6a−c, we plot bond breaking activation energies versus the values of the electron density, ρ(rc), and of the Laplacian of the density, −(1/4)∇2ρ(rc), at the bond critical points of the bonds being broken in the course of the dehydration reaction, for all the systems considered in this study. The correlation between the energy needed to break the C1−O bond and ρ(rc) is unmistakable (Figure 6a): the most effective catalyst is the one for which ρ(rc) is the smallest. This correlation extends itself to the Laplacian of the density, ∇2ρ(rc). According to the local virial theorem, ℏ2/4m·∇2ρ(rc) = 2G(r) + V(r), the Laplacian gauges the balance between the kinetic energy G(r) and the potential energy V(r). When ∇2ρ(rc) is substantially negative, one typically has a normal covalent bond. Positive ∇2ρ(rc) is usually associated with closedshell interactions (van der Waals, or ionic bonds) or Pauli repulsion. Similar behavior is observed for the C3−O (Figure 6b) and the C5−H (Figure 6c) bond breaking.

4. CONCLUSIONS Opening with a comment on methodology, our calculations show that nonembedded, relatively large, and flexible active site cluster models that are treated quantum mechanically yield 24422



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important conclusion of the current work is that, under the experimental conditions used by Williams et al., when the DMF conversion is carried out in HY, the uncatalyzed Diels−Alder reaction is the rate-limiting step, which explains why Dauenhauer and co-workers have reported that the rate of p-xylene production is independent of the density of Brønsted active sites.6 A schematic depiction of the overall energy profile along the HY reaction pathway is provided in Figure 7, in juxtaposition with the profile of the uncatalyzed process, and the computed profile for NaY. The alkali-exchanged Y zeolites do not exhibit notable catalytic activity in Diels−Alder cycloaddition, especially the smaller cations. The activity of the exchange cations as Lewis acids in the [4 + 2] Diels−Alder reaction is the result of considerations like geometry (viz., location of cation and positioning of adsorbates), charge transfer from the framework to the cations, and orbital mixing upon reactant binding. Charge population analysis of the bare active site shows screening of the charge of the cation, as anticipated. Li+ is screened the most, because it is more embedded in the framework and strongly interacts with the oxygen atoms. Interestingly, the binding process itself induces further significant electron density transfer from the framework oxygens to the cations. This inductive effect is stronger in the case of DMF binding and, in fact, it is strong enough to diminish the electron withdrawing capacity of the Lewis acid site to the extent that DMF participates in the cycloaddition nucleophilically and not electrophilically as one might have expected. The inductive effect drops in the order Li+ > Na+ > K+, but it is substantial for all three. Although the quantum of charge transfer in the course of cycloaddition is found to be always in the direction Δq‡(DMF) > 0 and Δq‡(eth) < 0, the reaction is not HOMOdonor-controlled, with “donor” standing for the DMF-active site complex. The calculated activation energies do not correlate with the HOMOdonor−LUMOacceptor energy gap and one must consider the comparable in magnitude LUMOdonor−HOMOacceptor energy gap to establish a correlation between activity and the energy gaps of the frontier molecular orbitals of proper symmetry. The reaction is, therefore, HOMOdonor- and LUMOdonor-controlled and, in this sense, one may speak of bidirectional electron flow. Looking ahead, we believe that a bifunctional catalyst, comprising both Lewis and Brønsted sites, could accelerate both the cycloaddition and the dehydrative aromatizationand by implication the overall processprovided that a zeolite framework is chosen so that the extra framework Lewis acid centers are less embedded and thus less screened by electron density transfer from the framework oxygen atoms.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This manuscript was based on work supported as part of the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001004. This research used resources of the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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REFERENCES

(1) Chang, C.-C.; Green, S. K.; Williams, C. L.; Dauenhauer, P. J.; Fan, W. Ultra-Selective Cycloaddition of Dimethylfuran for Renewable p-Xylene with H-BEA. Green Chem. 2014, 16, 585−588. (2) Brandvold, T. A. Carbohydrate Route to para-Xylene and Terephthalic Acid. US 2010/0331568 A1, December 30, 2010. (3) Román-Leshkov, Y.; Chheda, J. N.; Dumesic, J. A. Phase Modifiers Promote Efficient Production of Hydroxymethylfurfural from Fructose. Science 2006, 312, 1933−1937. (4) Román-Leshkov, Y.; Barrett, C. J.; Liu, Z. Y.; Dumesic, J. A. Production of Dimethylfuran for Liquid Fuels from Biomass-Derived Carbohydrates. Nature 2007, 447, 982−985. (5) Moliner, M.; Román-Leshkov, Y.; Davis, M. E. Tin-Containing Zeolites Are Highly Active Catalysts for the Isomerization of Glucose in Water. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 6164−6168. (6) Williams, C. L.; Chang, C.-C.; Do, P.; Nikbin, N.; Caratzoulas, S.; Vlachos, D. G.; Lobo, R. F.; Fan, W.; Dauenhauer, P. J. Cycloaddition of Biomass-Derived Furans for Catalytic Production of Renewable pXylene. ACS Catal. 2012, 2, 935−939. (7) Brion, F. On the Lewis Acid-Catalyzed Diels-Alder Reaction of Furan. Regio- and Stereo-Specific Synthesis of Substituted Cyclohexenols and Cyclohexadienols. Tetrahedron Lett. 1982, 23, 5299− 5302. (8) Chiappe, C.; Malvaldi, M.; Pomelli, C. S. The Solvent Effect on the Diels−Alder Reaction in Ionic Liquids: Multiparameter Linear Solvation Energy Relationships and Theoretical Analysis. Green Chem. 2010, 1330−1339. (9) Grieco, P. A.; Nunes, J. J.; Gaul, M. D. Dramatic Rate Accelerations of Diels−Alder Reactions in 5 M Lithium PerchlorateDiethyl Ether: The Cantharidin Problem Reexamined. J. Am. Chem. Soc. 1990, 112, 4595−4596. (10) Rideout, D. C.; Breslow, R. Hydrophobic Acceleration of Diels− Alder Reactions. J. Am. Chem. Soc. 1980, 102, 7816−7817. (11) Houk, K. N.; Strozier, R. W. Lewis Acid Catalysis of Diels− Alder Reactions. J. Am. Chem. Soc. 1973, 95, 4094−4096. (12) Blake, J. F.; Jorgensen, W. L. Solvent Effects on a Diels−Alder Reaction from Computer Simulations. J. Am. Chem. Soc. 1991, 113, 7430−7432. (13) Blake, J. F.; Lim, D.; Jorgensen, W. L. Enhanced Hydrogen Bonding of Water to Diels−Alder Transition States. Ab Initio Evidence. J. Org. Chem. 1994, 59, 803−805. (14) Furlani, T. R.; Gao, J. Hydrophobic and Hydrogen-Bonding Effects on the Rate of Diels−Alder Reactions in Aqueous Solution. J. Org. Chem. 1996, 61, 5492−5497. (15) Frising, T.; Leflaive, P. Extraframework Cation Distributions in X and Y Faujasite Zeolites: A Review. Microporous Mesoporous Mater. 2008, 114, 27−63. (16) Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two

ASSOCIATED CONTENT

* Supporting Information S

Additional material is included in the Supporting Information. Table S1 contains proper symmetry HOMO and LUMO of the ethylene- and DMF-active site complexes. Table S2 contains charge changes for the cations in the Diels−Alder transition state. Table S3 contains data that pertain to the fitting of eq 1 in the text. Figure S1, shows the orientation of adsorbates at the active site. Figure S2, shown the mechanism and associated energy profile for protonation of ethylene in HY. Figure S3, shows ground and transition state geometries for the three elementary steps in the dehydration of the cycloadduct on NaY. This material is available free of charge via the Internet at http://pubs.acs.org. 24423



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New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215−241. (17) Fitch, A. N.; Jobic, H.; Renouprez, A. Localization of Benzene in Sodium-Y-Zeolite by Powder Neutron Diffraction. J. Phys. Chem. 1986, 90, 1311−1318. (18) Forano, C.; Slade, R. C. T.; Andersen, E. K.; Andersen, I. G. K.; Prince, E. Neutron Diffraction Determination of Full Structures of Anhydrous Li−X and Li−Y Zeolites. J. Solid State Chem. 1989, 82, 95− 102. (19) Lim, W. T.; Choi, S. Y.; Choi, J. H.; Kim, Y. H.; Heo, N. H.; Seff, K. Single Crystal Structure of Fully Dehydrated Fully K+Exchanged Zeolite Y (FAU), K71Si121Al71O384. Microporous Mesoporous Mater. 2006, 92, 234−242. (20) Feuerstein, M.; Lobo, R. F. Characterization of Li Cations in Zeolite LiX by Solid-State NMR Spectroscopy and Neutron Diffraction. Chem. Mater. 1998, 10, 2197−2204. (21) Bader, R. F. W. A Quantum Theory of Molecular Structure and Its Applications. Chem. Rev. (Washington, DC, U. S.) 1991, 91, 893− 928. (22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al. Gaussian 09, revision A.2; Gaussian Inc.: Wallingford, CT, 2009. (23) Biegler-König, F.; Schönbohm, J.; Bayles, D. AIM2000 - A Program to Analyze and Visualize Atoms in Molecules. J. Comput. Chem. 2001, 22, 545−559. (24) Kim, H. S.; Bae, D.; Lim, W. T.; Seff, K. Li+ Exchange into Zeolite Na−Y (Fau) from Aqueous Methanol. Single-Crystal Structures of Fully Dehydrated Li,Na−Y. J. Phys. Chem. C 2012, 116, 9009−9018. (25) Hriljac, J. A.; Eddy, M. M.; Cheetham, A. K.; Donohue, J. A.; Ray, G. J. Powder Neutron Diffraction and 29Si MAS NMR Studies of Siliceous Zeolite-Y. J. Solid State Chem. 1993, 106, 66−72. (26) Fermann, J. T.; Moniz, T.; Kiowski, O.; McIntire, T. J.; Auerbach, S. M.; Vreven, T.; Frisch, M. J. Modeling Proton Transfer in Zeolites: Convergence Behavior of Embedded and Constrained Cluster Calculations. J. Chem. Theory Comput. 2005, 1, 1232−1239. (27) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions. J. Phys. Chem. B 2009, 113, 6378−6396. (28) Gilchrist, T. L.; Storr, R. C. Organic Reactions and Orbital Symmetry, 2nd ed.; Cambridge University Press: New York, 1979. (29) Domingo, L. R.; Aurell, M. J.; Pérez, P.; Contreras, R. Quantitative Characterization of the Local Electrophilicity of Organic Molecules. Understanding the Regioselectivity on Diels−Alder Reactions. J. Phys. Chem. A 2002, 106, 6871−6875. (30) Houk, K. N. Generalized Frontier Orbitals of Alkenes and Dienes. Regioselectivity in Diels−Alder Reactions. J. Am. Chem. Soc. 1973, 95, 4092−4094. (31) Woodward, R. B.; Hoffmann, R. The Conservation of Orbital Symmetry. Angew. Chem., Int. Ed. 1969, 8, 781−932. (32) Fukui, K. Role of Frontier Orbitals in Chemical Reactions. Science 1982, 218, 747−754. (33) Fukui, K.; Inagaki, S. An Orbital Interaction Rationale for the Role of Catalysts. J. Am. Chem. Soc. 1975, 97, 4445−4452. (34) Inagaki, S.; Fujimoto, H.; Fukui, K. Orbital Interaction in Three Systems. J. Am. Chem. Soc. 1976, 98, 4693−4701. (35) Inagaki, S.; Fujimoto, H.; Fukui, K. Orbital Mixing Rules. J. Am. Chem. Soc. 1976, 98, 4054−4061. (36) Cant, N. W.; Hall, W. K. Studies of the Hydrogen Held by Solids: XXI. The Interaction Between Ethylene and Hydroxyl Groups of a Y-Zeolite at Elevated Temperatures. J. Catal. 1972, 25, 161−172. (37) Namuangruk, S.; Pantu, P.; Limtrakul, J. Investigation of Ethylene Dimerization over Faujasite Zeolite by the Oniom Method. ChemPhysChem 2005, 6, 1333−1339.

(38) Nikbin, N.; Do, P. T.; Caratzoulas, S.; Lobo, R. F.; Dauenhauer, P. J.; Vlachos, D. G. A DFT Study of the Acid-Catalyzed Conversion of 2,5-Dimethylfuran and Ethylene to p-Xylene. J. Catal. 2013, 297, 35−43. (39) Knifton, J. F.; Sanderson, J. R.; Dai, P. E. Olefin Oligomerization via Zeolite Catalysis. Catal. Lett. 1994, 28, 223−230. (40) O’Connor, C. T.; Kojima, M. Alkene Oligomerization. Catal. Today 1990, 6, 329−349.

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